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Tapered waveguides are useful for efficiently interconnecting waveguides that support modes of different dimensions. Semiconductor laser chips typically contain waveguides that tightly guide the optical mode, while the optical fiber that transmits the laser light guides weakly so that the modes match poorly if the laser is connected directly to the fiber. Many approaches have been used to improve the efficiency of such interconnections. Vertical, lateral, and refractive index tapers were demonstrated by * R. Logan, U.S. Pat. No. 3,978,426, Aug. 31, 1976, and * R. K. Winn et al., IEEE Trans. Microwave Theory and Techniques MTT23 92 (1975), and * P. G. Suchoski Jr., et al., J. Light. Technology, LT5 1246 (1987). Both waveguide claddings and fibers have been tapered; see * J. Hammer, U.S. Pat. No. 4,773,720, Sep. 27, 1988, and * H. Schneider, U.S. Pat. No. 4,795,228, Jan. 3, 1989. Multiple stage tapers in the vertical and longitudinal directions were shown by * T. Koch, U.S. Pat. No. 4,932,032, Jun. 5, 1990 and * P. Melman, U.S. Pat. No. 5,261,017, Nov. 9, 1993. Pairs of coupled waveguides have been shown with single and multiple horizontal and vertical tapers by * Y. Shani et al., Appl. Phys. Lett. 55 2389 (1989), * E. Kapon, U.S. Pat. No. 5,078,516, Jan. 7, 1992, * Zengerle et al., Elect. Lett. 28 631 (1992), * B. Stegmueller, U.S. Pat. No. 5,199,092, Mar. 30, 1993 and * R. Smith et al., IEEE Photon. Technology Lett. 8 1052 (1996). Segmented tapers are shown by Z. Weissman et al., J. Light. Technology, 11 1831 (1993), and R. Adar, U.S. Pat. No. 5,577,141, Nov. 19, 1996. A good deal of effort has also been expended recently on optimizing integrated tapers in semiconductor lasers. However, the practical constraints that apply to any specific implementation tend to require different coupling approaches depending on both materials and geometry: the processes used to manufacture and package a product must be compatible with each other, with the materials used, and with the component layout. A unique geometry of taper and waveguides is required to optimize the coupling between a laser chip that is to be butt coupled in a hybrid configuration to a planar waveguide chip.
According to the invention, a waveguide is fabricated on a substrate with a protrusion of the core material extending from the cladding surface. The protrusion and the waveguide are coupled for refracting light propagating along an axis of the waveguide. In a first method, the protrusion is fabricated by etching across the waveguide propagation axis, through core and cladding materials to reveal a second etching surface that is preferentially etched. In one variation, the cladding material is etched faster than the core, forming a projection from the surface. In another variation, the core material is etched faster than the cladding, forming an indentation in the surface. The radius of curvature of the protrusion depends on the core dimensions, and may be changed by changing the waveguide dimensions. Two independent radii of curvature are formed from a waveguide with a core of rectangular cross section; these two different radii may be used to couple an astigmatic beam efficiently into the waveguide. The waveguide may have a transition from the rectangular cross section to another cross section within a short distance of the surface. The protrusion may also be covered with a material of dissimilar index of refraction. In a specific embodiment, the waveguide is fabricated from silica structures doped with Ge within the core region, using reactive ion etching to reveal the second etching surface, and a liquid etchant to preferentially etch the protrusion. A heating step such as by laser illumination may also be used to smooth and reshape the surface of the protrusion.
The invention will be better understood upon reference to the following detailed description in connection with the accompanying drawings.